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PHYS 157 > Thermodynamics > Flashcards

Thermodynamics Flashcards

random things I may need to know (90 cards)

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1
Q

difference between heat and temperature?

A

heat: refers to energy transfer caused by dif temps.

temperature: form of measurement. associated with the sense of touch and related to kinetic energies of the molecules in the material.

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2
Q

0th law of thermodynamics

A

if C is initially at eqlbm w both A & B, then A & B are both at eqlbm with each other

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3
Q

Actual temperature vs temperature interval

A

actual temp: written as __°C (blank degrees Celsius)

temp interval: __ C° ( blank Celsius degrees); its a difference/ change in temp

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4
Q

when volume is constant, pressure is directly proportional to…

A

Kelvin temperature so T1/T2 = P1/P2

ratio of two temps in Kelvin equals the ratio of corresponding pressures

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5
Q

To find the temp of T at the triple point of water…

A

Use the temp-pressure ratio.

T/T_triple = P/P_triple

T_triple = 273.16K

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6
Q

Linear Thermal Expansion eqn

A

ΔL = αL_oΔT

ΔL = change in length
α = coefficient of linear expansion
L_o = original length
ΔT = temp change

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7
Q

what is α and what are the units?

A

α is the coefficient of linear expansion, it is constant and is based on material.

units are K^-1 or C°^-1

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8
Q

Volume Thermal Expansion Eqn

A

ΔV = ßV_oΔT

ΔV= Change in Volume
ß = coefficient of volume expansion
V_o = Original volume
ΔT= change in temp

same units as linear expansion

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9
Q

When do you use volume expansion versus linear expansion

A

Volume expansion = liquids

linear = rods, change in length, solids

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10
Q

How to find ß when given the α of a material?

A

ß = 3α

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11
Q

Young’s modulus eqn - this involves tension

A

Y = (F/A)/(ΔL/L_o)
= (FL_o)/(AΔL)

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12
Q

to calculate thermal strain…

A

ΔL/L_o = αΔT

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13
Q

To solve thermal stress F/A…

A

F/A = -YαΔT

F = force needed to keep length of rod
A = cross sectional area of material
Y= young’s modulus
α = coefficient of linear expansion
ΔT= change in temp

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14
Q

what is young’s modulus and the units?

A

Young’s modulus is a measure of stiffness of a material

it is Tensile Stress/ Tensile Strain

Low Y =more flexible
High Y = more stiff

Units are Pa or N/m^2

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15
Q

what is tensile stress eqn

A

stress is force applied per area (F/A)

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16
Q

what is tensile strain eqn

A

deformity / change in shape due to force (ΔL/L_o)

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17
Q

What’s Hooke’s Law?

A

the greater the deforming forces, the greater the resulting deformation

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18
Q

tensile stress vs compressive stress

A

tensile stress is when force stretches/ elongates material

compressive stress is when force compresses/ shrinks the material

in general tensile is stretching

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19
Q

when temperature goes down, what force /stress is needed to maintain length?

A

contraction would happen when ΔT is negative so tensile force and stress is needed to pull the material to keep the length

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20
Q

when temp goes up, what force/stress is needed to maintain length?

A

material expands when ΔT is positive so required force and stress is compressive in order to maintain length

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21
Q

elastic modulus

A

property of material of which object is made

the ratio is stress/ strain

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22
Q

what is elasticity? elastic v plastic?

A

it depends on the type of material and not size

elastic: return to OG state after stress is removed

plastic: remains deformed after stress is removed

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22
Q

what is Bulk strain

A

ΔV/V_o

change in volume per unit volume

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23
Q

pressure in a fluid eqn

A

P=F/A

P = pressure in a fluid
F = force
A = area over which force is exerted

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24
what is the bulk modulus for compression?
bulk stress/ bulk strain = -ΔP/(ΔV/V_o) ΔP= additional pressure on object ΔV = change in volume V_o= OG volume *minus sign is bc inc in pressure always leads to dec in volume
25
pressure is inversely proportional to...
volume
26
stress is proportional to...
strain
27
heat flow/ heat transfer
energy transferred only by ΔT
28
equation for heat required to change temp
Q=mcΔT m = mass of material c = specific heat of material ΔT = change in temperature
29
high v low specific heat
high specific heat needs more heat to change temp low specific heat heats up and slows down quicker
30
heat required to change temp of a certain number of moles
Q =nCΔT n = # of moles C = molar mass times specific heat capacity ΔT = change in temp
31
heat of fusion/ latent heat of fusion (solid to liquid) Q needed
Q = +/- mL_f m = mass of material L_f = latent heat
32
what is heat of vaporization
the heat per unit mass to change liquid to gas
33
heat current in conduction/ rate of heat flow eqn
H = kA(Th-Tc)/L H= heat current k= thermal conductivity of rod material A= cross-sectional area of rod Th-Tc = temp of hot and cold rod
34
thermal resistance of slab in correlation to H is
H = A(Th-Tc)/R R= thermal resistance A = cross-sectional area
35
thermal resistance is given by
L/k L= thickness of rod k = thermal conductivity
36
materials with low k...
are good insulators
37
materials with a large k...
are good conductors of heat
38
when doubling thickness what happens to R
R doubles
39
what is emissivity?
e is a measurement of how efficiently a surface emits thermal radiation compared to a perfect black body at the same temperature
40
what is a black body?
A black body is an idealized object that absorbs all incident radiation and emits the maximum possible radiation at a given temperature
41
heat current of radiation equation aka Stefan-Boltzmann Law
H = AeσT^4 H = rate of heat flow A = surface area of emitting surface e = emissivity of surface σ = Stephan Boltzmann constant = 5.67037442 * 10^-8 W/(m^2*K^4) T = absolute temp of surface
42
Net Heat current in radiation
H_net = H-emitted - H-absorbed = AeσT^4 - AeσT_s^4 H_net = Aeσ(T^4 - T_s^4) T = temperature of the radiating surface T_s = temperature of the surroundings or the object with which the surface is radiating to
43
Wien's Law
λ_max = (2.9Kmm)/T λ_max = peak wavelength
44
what is the fraction of power in a certain range of wavelength?
the fraction of area under the graph in that range
45
when two objects are at equilibrium...
H_in = H_out H_in = rate at which sunlight/ source of heat is absorbed H_out = rate at which heat is absorbed
46
what is albedo
albedo is the overall average reflection coefficient for solar radiation incident on an object
47
What is H_in due to sunlight
H_in = I_sc*pi*r_e^2 (1-a_e) I_sc = solar constant r_e = radius of earth a_e = albedo
47
what is solar constant
the intensity of sunlight, I_sc
48
What is the temperature equilibrium equation for radiation?
T_e = [(1-a_e)I_sc/(4e_eσ)]^1/4 T_e = temperature eqlbm a_e = albedo I_sc = solar constant e_e = emissivity σ = stephan boltzmann constant
49
what is intensity formula
power/area
50
what is total power in terms of intensity
power = intensity * area
51
H_in for earth
H _in = H_sun (r_e^2)/(4R_s-e^2)(1-a_e) r_e = radius of earth R_s-e = distance from sun to earth a_e= albedo of earth
52
if surface moves towards particle...
surface does work on particle during collision
52
if surface is stationary...
work is done on the surface by the particle
53
in a PV- diagram, if the volume expands....
work is positive - work is also the area under the curve
54
in a PV - diagram, if the volume decrease...
work is negative
55
in a PV- diagram, if work is done at constant pressure...
work is positive and is equal to p=(V_2-V_1)
56
path
a series of intermediate states that get passed thru when a thermodynamic system changes from an initial state to a final state
57
is work path dependent
yes it is - depends not only on the final and initial state but also the intermediate states - Q is also path dependent
58
First Law of Thermodynamics
ΔU = Q - W
59
is ΔU path dependent
no it is independent from the path; only depends on initial and final state
60
Adiabatic
no heat transfer into or out of system; Q = 0 therefore: ΔU = -W
61
when system expands adiabatically, work...
is done on its surroundings work is pos = -ΔU
62
when system compresses adiabatically, work...
is done on the system by surroundings W is neg = + ΔU
63
Isochoric
constant volume process W = 0 therefore ΔU = Q
64
For isochoric processes, work is....
not done on surroundings, it doesn't exist.
65
Isobaric
constant - pressure process W = p(V2-V1) none of the qualities are zero
66
Isothermal
constant temp ΔU = 0 therefore: W = Q
67
For an ideal gas Cp =
Cp = Cv + R Cp = molar heat capacity at constant pressure Cv= molar heat capacity at constant volume R = gas constant (8.314 J/(mol*K))
68
Ratio of heat capacities
γ = Cp/Cv
69
at constant pressure for an ideal gas ΔU...
ΔU = nCpΔT
70
at constant volume for an ideal gas ΔU...
ΔU = nCvΔT
71
thermal efficiency of an engine
e = W/Q_H = 1 - |Qc/Q_H| e = thermal efficiency W = work done by engine Qc= heat rejected by engine Q_H = heat absorbed by engine
72
Steps in an Otto Cycle
cyclic process (starts and ends at the same place) 1.compressed adiabatically 2. heated at constant volume 3. expands adiabatically 4. cooled at constant volume
73
thermal efficiency in otto cycle
e = 1- 1/(r(γ -1)) r= compression ratio γ = ratio of heat capacities
74
Steps in a Diesel Cycle
1. compressed adiabatically 2. heated at constant pressure 3. expanded adiabatically 4. cooled at constant pressure
75
work for an engine
W = Q_H + Qc W = |Q_H| - |Qc|
76
work for refrigerator
|W| = |Q_H| - Qc
77
what is the equation for CoP?
K = |Qc|/|W| = |Qc| / [ |Q_H| - |Qc| ] K = coefficient of performance of a refrigerator W = work input of refrigerator Q_H = Heat rejected from refrigerator Qc = heat removed from inside of refrigerator
78
Irreversible Processes
proceeds spontaneously in one direction but not the other
79
Second Law of Thermodynamics - Kelvin Plank Statement "Engine" Statement
heat cannot completely be converted into mechanical work and be a cyclic process
80
Second Law of Thermodynamics - "refrigerator" statement
heat does not go from cold to hot naturally without assistance
81
entropy
the randomness of molecules; the more entropy, the more energy there is
82
total entropy change during a reversible isothermal process
ΔS = Q/T
83
total entropy for a Carnot engine
zero
84
is entropy path dependent
no; only needs initial and final state
85
Second Law of Thermodynamics
no process is possible in which total entropy decreases
86
Carnot Engine
maximum possible efficiency of an engine on a temp v entropy graph its a square
87
Steps of the Carnot Engine
1. expands isothermally -> absorbs heat Q_H 2. expands adiabatically -> Tc drops 3. compresses isothermally -> rejects |Qc| 4. expands adiabatically to OG state

PHYS 157 (3 decks)

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